The present disclosure relates generally to turbochargers, and relates more particularly to exhaust gas-driven turbochargers having a variable turbine nozzle in the form of an array of vanes that are pivotable about their axes between a closed position and an open position, and any position therebetween, for regulating the flow of exhaust gas through the nozzle to the turbine wheel.
In a typical turbocharger as described above, the vanes are rotatably mounted on a generally annular nozzle ring disposed in the turbocharger surrounding a central axis about which the turbine wheel rotates. The vanes extend between the nozzle ring and an opposite wall defined by an insert disposed in the turbine housing. The nozzle flow path extends between the nozzle ring and the insert, and thus the exhaust gas flows from the turbine housing chamber, radially inwardly between the vanes, and into the turbine wheel. By pivoting the vanes, the effective flow area of the nozzle is varied, thereby regulating the flow of exhaust gas to the turbine wheel.
The vanes are typically mounted on the nozzle ring by way of shafts affixed to one end of the vanes and received in bearing apertures that extend through the nozzle ring. The portions of the vanes exposed to exhaust gas flow in the nozzle are shaped as airfoils whose opposite ends are closely proximate to the faces of the nozzle ring and the opposite insert, respectively. Ideally, the clearance between each end of the airfoil portion and the adjacent face should be zero so that exhaust gas cannot leak through the clearance. However, in practice it is not possible to have zero clearance, or even a very small clearance, because binding would occur between the ends of the airfoil portion and the adjacent faces of the nozzle ring and insert. This is particularly true in view of the thermally induced deformations of the various parts that take place during turbocharger operation. The various parts undergo thermal growth and contraction at different rates and in different amounts.
Accordingly, in practice, the nominal vane clearances typically are designed to be relatively large so as to avoid any possibility of the vanes binding. While the clearances may be smaller at some operating conditions, by design there is still a considerable clearance over the entire expected range of operating conditions. These substantial clearances are known to cause a loss in turbine efficiency. However, it has generally been assumed that such efficiency loss is unavoidable because of the need to prevent any possibility of vane binding.